1. Field of the Invention
This invention relates to an electrohydraulic control system for controlling a hydraulic drive element such as a cylinder of a press, and, more particularly, to such a control system having a stepping drive operative to move a link which in turn is coupled to move a spool member of each of a pilot valve and a main valve for controlling the delivery of pressing fluid to the drive element.
2. Description of the Prior Art
Various constructions of electrohydraulic control systems are known in this art as proportional dividers because of the nature of the operation of the system. In the known system, the means for actuating a pilot valve spool can take the form either of two proportional magnets which engage opposite end faces of the spool or of a single such magnet in which event the spool is spring biased at one end. To actuate the proportional valve, signals in a digital form must be converted into analogue voltage values. The conversion of signals from a digital to analogue is a very elaborate procedure which cannot be accomplished without deviations particularly in the degradtion performance of certain control functions. Moreover, in the conversion process, operating points cannot be reproduced in a manner which is free from deviations. Such deviations are a severe disadvantage particularly in the control of a numerically controlled hydraulic press because the unavoidable deviations lead to variations in the positioning of the press ram.
It also has been found in practice that proportional valves manifest a temperature dependent behavior pattern. Consequently, a proportional valve cannot be used to control a hydraulic press with a constant accuracy throughout a working day; thus also undesired variations to the quality of the end product. Also, the known proportional valves are sensitive to dirt, and systems which use proportional valves cannot operate without losses. Still a further disadvantage arises out of the absence of a feedback between set values and actual values when using proportional valves.